This invention relates to optical pick-up heads using vertical surface cavity emitting lasers. More particularly, the invention relates to monolithically integrated laser/detector chips for use in the high-density optical data storage applications.
Optical media providing digital data storage include Compact Disks (CDs), Digital Versatile Disks (DVDs), Mini Discs (Mds). High-density storage in these applications becomes important for music, video, and computer software. Optical pickup heads are used for reading information stored on a surface of such media.
Conventional optical pick-up heads consist of several components: a laser diode, a beam splitter, diffraction grating, collimating lens, objective lens, photodiode array and, possibly, additional holographic optical elements. By requiring a multitude of separate components, the optical pick-up head becomes thick, large and expensive. For the same reasons, optical alignment of the system during its assembly becomes problematic.
One will appreciate that there are two possible opportunities to obtain a small, reliable and low-cost optical pick-up that is easily capable of being mass-produced. First, the sizes of the light-emitting and light-detecting units should be reduced and, preferably, these elements should be integrated into a single semiconductor chip. Second, the optical design should be simplified to reduce the number of required elements.
One advantageous solution for decreasing the laser size is to replace an edge-emitting laser currently used in an optical disk pickup with a Vertical Cavity Surface Emitting Laser (VCSEL). The lower size limit for the fabrication of edge-emitting lasers is 50 microns in width and 100-200 microns in length. In comparison, VCSELs can be made to minute circular units of less than 10 microns in diameter. Current VCSELs emit at wavelengths between 650 nm and 980 nm. They are made of GaAs/GaAlAs, InGaAs, GalnP and AlGaAs, InP, having a sandwiched structure of multi-layer reflective surfaces between which multiple quantum-wells are formed. The broad range of available VCSEL wavelengths makes it a good candidate for use in both DVD and CD optical pickups.
Since the date of the VCSEL invention, some designs of optical pickups employing a VCSEL as a light source have been proposed. For example, U.S. Pat. No. 5,757,741 to Jiang et al., which is incorporated herein by reference, describes a CD ROM head using VCSELs. In this design as well as in other known prior art devices containing VCSELs, the conventional optical schematic is used. Because of the presence of beam splitters or holographic elements, the light-detecting parts should be distanced from VCSEL. Therefore, the replacement of edge-emitting lasers with VCSELs in these prior art devices has not brought a significant reduction of the size of the optical head and did not simplify its optical design.
It is also known that a group of inventors from the Sony Corporation is developing a promising technique that allows the use of a very simple optical design. Variants of their method have been disclosed by Sahara et al. in U.S. Pat. No. 5,568,463, by Doi et al. in U.S. Pat. No. 5,883,913, and by Mizuno et al. in the Japan Journal of Applied Physics, 1999, vol. 38, p.2001, all of which being incorporated herein by reference. The main idea of these designs is based on the phenomenon that the diameter of the light beam returning back after reflection from the disk surface is much larger than that of the initial beam emitted by the laser. As a result, a detector can be placed in close vicinity to a laser. In fact, the two can be disposed on the same optical axis. With this, such an optical schematic can contain only focusing elements and can avoid beam splitters or holographic elements. Laser and detectors are monolithically integrated into a single element.
Unfortunately, there are at least three disadvantages relating to these designs proposed by Sony. All are connected with the use of an edge-emitting laser as a light source. First, because of the comparably large size of such lasers, the dimensions of a final chip containing the lasers becomes resultantly large. Second, such optical designs are limited to detecting less than half of returned light. Third, the geometry of monolithic integrated chips is complicated and requires a large number of operations during its production.
Advantageously, the present invention sets forth with the basic object of providing improved designs for optical pickup heads.
A more particular object of the invention is to provide a monolithic method for making optical pickup heads that are thin.
Another object of the invention is to provide a monolithic method for making integrated VCSEL/detector chips for optical pickup heads that has data reading, tracking, focusing, and power monitoring capabilities.
A still further object of the invention is the provision of an optical pickup and servo control device that accomplishes the foregoing objects while materially reducing the cost thereof relative to prior art optical pickup heads.
A first preferred embodiment in accordance with the present invention comprises an integrated laser/detector chip, a stepped micro-mirror and an optical means for focusing laser light on a disk. The integrated chip contains a group of VCSELs that each act as means for generating an optical beam and that have a small aperture in the range of a few microns, photodiodes placed on the top of each VCSEL surrounding its aperture, and photodiodes for monitoring the power of each VCSEL from its bottom. This chip provides the system with data signals as well as with servo signals. The stepped micro-mirror provides the detection of focusing error signal (FES) employing a spot-size method. The tracking error signal (TES) is detected either with a push-pull method or with a 3-beam (outrigger) method. The monitoring of VCSEL power allows it to maintain a desirable level using power feedback control.
In a second embodiment of the invention, three additional VCSELs emitting at a wavelength of approximately 650 nm are added to the chip to provide data reading either from a CD or a DVD.
In a third embodiment of the invention, the integrated chip contains a wedge-like pyramid coated by a metal that serves as micro-mirrors.
In a fourth embodiment of the invention, an edge-emitting laser is inserted into the design of, for example, the third embodiment to provide for a recording of data on a disk surface.
In fifth and sixth embodiments, additional red VCSELs are used in the design of the third embodiment to provide data reading from a CD or a DVD. These two embodiments use different methods for DVD tracking.
With a plurality of embodiments of the present invention for a Monolithic VCSEL-based Optical Pickup and Servo Control Device described, one will appreciate that the foregoing discussion broadly outlines the more important features of the invention merely to enable a better understanding of the detailed description that follows and to instill a better appreciation of the inventors"" contribution to the art. Before an embodiment of the invention is explained in detail, it must be made clear that the following details of construction, descriptions of geometry, and illustrations of inventive concepts are mere examples of the many possible manifestations of the invention.